1. Field of the Invention
This invention relates to a signal separator and a bandpass filter that are capable of handling high power radio frequency (rf) signal energy in television broadcast antenna installations.
2. Description of the Prior Art
The advent of digital television (DTV) has resulted in a need for a station to broadcast both a national television standard code (NTSC) signal for reception by NTSC sets and a DTV signal for reception by DTV sets. A common practice is to mount a DTV antenna to the existing tower upon which is mounted the NTSC antenna. A single transmission line extends up the tower for the purpose of feeding the NTSC signal from a transmitter to the NTSC antenna. Although the transmission line may be either a waveguide or a coaxial structure, a coaxial structure is preferred because of the additional wind loading on the tower and group delay distortion on the signal that results from a waveguide structure.
A separate transmission line could be added to feed the DTV signal up the tower to the DTV antenna. However, it is preferable to use the single existing transmission line to feed both the NTSC and the DTV signals to their respective antennas as this is less expensive and does not add wind load. For example, U.S. Pat. No. 5,774,193 uses a signal combiner to combine the NTSC and DTV signals to form a composite signal that is fed up the transmission line. A signal separator disposed at the upper end of the transmission line separates the composite signal into the DTV signal and NTSC signal for application to the DTV and NTSC antennas. The signal separator is formed of a high pass filter and a low pass filter. The high pass filter passes an ultra high frequency (UHF) DTV signal to the DTV antenna, but rejects a very high frequency (VHF) NTSC signal. The low pass filter passes the VHF NTSC signal to the NTSC antenna, but rejects the UHF DTV signal. The high and low pass filter separator may provide adequate bandwidth and rejection for the case of the DTV signal and the NTSC signal being in two different frequency bands. However, it does not provide adequate rejection for the case where the carrier frequencies of the two signals are relatively close together as, for example in the same frequency band.
Accordingly, there is a need for a signal separator that has adequate bandwidth and rejection for the case where the carrier frequency difference of signals to be separated is relative small. There is also a need for filter assemblies that can be used in such a separator.
The present invention satisfies the aforementioned needs with a signal separator that uses separate bandpass filters for each signal that is to be separated from a composite signal. Each bandpass filter is tuned to the carrier frequency of a different one of the signals. The filter assembly of the invention includes a coaxial transmission line having a hollow outer conductor and an inner conductor disposed within the outer conductor. One or more electrically conductive elements are disposed within the outer conductor and coupled mechanically and electrically to the outer conductor. In some preferred embodiments, each of the the electrically conductive elements is a plate that includes a metallic annulus and a metallic post that is diametrically disposed with respect to the annulus and that is coupled to the inner conductor. The number of plates and the distance between adjacent ones of the plates are determinative of the frequency of operation, bandwidth and rejection capability of the filter.
In one embodiment of the invention, the transmission line has a plurality of segments and one of the plates is disposed at an interface of adjacent ones of the segments. This embodiment takes advantage of flange connectors at the ends of each segment to form mechanical and electrical connection to one of the plates as well as to the outer conductors of adjacent segments.
Unlike a waveguide approach, the coaxial filter assemblies of the present invention are suitable for handling the pressurization of a coaxial line without changing the response of the separator. Also, the filter assemblies and separator are stable over a large temperature range that permit mounting the separator on the outside of the tower or other location that is exposed to ambient.